Terrestrial analogs for interpretation of infrared spectra from the Martian surface and subsurface: Sulfate, nitrate, carbonate, and phyllosilicate‐bearing Atacama Desert soils

Abstract

Hyperarid (<2 mm y −1 ) Atacama Desert soils of northern Chile are known for nitrate, sulfate, and chloride accumulations. Atacama and Mars soils have similar sulfate concentrations; possess phyllosilicates (e.g., smectite) and minor carbonate. Nitrate has not been detected on Mars, but its presence has been proposed. The similar compositions of Atacama and Mars soils have prompted the visible‐infrared (0.35–25 μ m) investigation of Atacama soils as Mars analogs. Results from this work determined the best infrared features for detecting sulfate, nitrate, carbonate, and phyllosilicate on Mars. The fundamental region (>6.5 μ m) was not suited for salt and phyllosilicate detection because of overlapping spectra from primary silicates (e.g., feldspar), water and carbon dioxide. The visible near‐infrared (0.35–2.5 μ m) region was suited for detecting carbonate, nitrate, gypsum water of hydration, and phyllosilicate hydroxyls without interference from primary silicates. However, gypsum water of hydration features can obscure phyllosilicate hydroxyl, carbonate and nitrate, features if gypsum levels are high. Overtone/combination absorption features in the midinfrared were determined to be the best indicators of sulfate (4.48–4.70 μ m), nitrate (4.12 μ m), and carbonate (3.98 μ m) because interferences from overlapping primary silicate and water features are not present in this region. Interferences from CO 2 and thermal emission effects in the overtone/combination region are possible but may be minimized by corrective techniques. Infrared analysis of Atacama Desert soils can provide insight into the spectral search of sulfate, nitrate, carbonate, and phyllosilicate containing soils on Mars.